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Publication numberUS4944975 A
Publication typeGrant
Application numberUS 07/251,773
Publication dateJul 31, 1990
Filing dateOct 3, 1988
Priority dateOct 3, 1988
Fee statusLapsed
Also published asCA2000085A1, DE68925777D1, DE68925777T2, EP0364811A2, EP0364811A3, EP0364811B1
Publication number07251773, 251773, US 4944975 A, US 4944975A, US-A-4944975, US4944975 A, US4944975A
InventorsM. Lana Sheer
Original AssigneeE. I. Du Pont De Nemours And Company
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Composite coil forms for electrical systems
US 4944975 A
An electrical coil form having a UL 1446 rating greater than 200° C. is formed from either a thermoplastic or thermosetting resin material reinforced with continuous or long chopped or stretch broken aramid or glass fibers or aramid paper.
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What is claimed is:
1. A coil form for an electrical device comprising:
a structure of an aramid prepreg impregnated with an electrically insulating resin, said structure having a longitudinal passage therethrough, a continuous outer peripheral surface forming support for a coil wound thereon and structural stability and a UL Standard 1446 rating of greater than 200° C., said aramid prepreg having fibers greater than 1/4 inch in length, said aramid prepreg being an aramid paper which is a blend of Kevlar® aramid and a Nomex® aramid in an 85% to 15% by weight ratio, respectively.

This invention relates to electrical device coil forms and, more particularly, it relates to a coil form produced from fiber reinforced resin material.

The market segment for electrical devices such as for windings of motors, transformers and solenoids is increasingly moving to miniaturization of such devices. This in turn leads to a rise in internal equipment operating temperature resulting in a need for higher temperature ratings on insulation materials used for these applications.

Coil forms fashioned from Nomex® aramid paper, a high temperature resistant paper with excellent physical and electrical properties are known for use in high temprature (greater than 200° C. rating of UL Standard 1446) wherein a coil of wire is wound over the Nomex® paper coil form placed on a mandrel to support the form. However, this product does not exhibit a high degree of structural strength and the wire coil must be wound fairly loosely over the Nomex® paper coil form or the Nomex® will implode after the wire windings are in place and the winding mandrel is removed. The use of this coil form system is labor intensive, requires more wire than necessary and provides lower transformer efficiency.

Insulation systems approved to UL Standard 1446 are rated by subjecting them to cycles of thermal aging, low temperature shock, humidity and vibration. At the end of each cycle the systems are checked at 600 volts for short circuits, with the cycle being repeated until the insulation breaks down. Test voltages may be higher for systems with higher operating voltages. The temperature rating assigned to an insulation system will depend on the service life of the equipment at different aging temperatures.


In order to overcome the above-noted deficiencies, a coil form has been developed which has high structural stability at a UL Standard 1446 rating of greater than 200° C. and comprises a structure of fiber reinforced resin matrix material having longitudinal passage therethrough. The outer peripheral surface of the structure forming a support for a wire coil wound thereon.

Suitable materials which may be used as the resin matrix include electrically insulating thermoplastic or thermoset resins such as polyethylene terephthalate, 6,6-nylon or electrical grade epoxy.

The resin of choice is reinforced with fibers such as, for example, glass and aramid fibers which may be continuous, long fiber discontinuous such as chopped or randomly broken, but in any event greater than 1/4" in length. The fiber volumes preferably are in the range of from about 15 to about 70% and more preferably in the range of from 20 to 50%. The coil forms can be made by any known process for making such forms as by braiding and filament winding of resin coated materials or by pultrusion methods or indeed by hand lay-up techniques well known in the art.

Another preferred embodiment is an aramid prepreg based on an electrically insulating resin.


The drawing illustrates a perspective view of a coil form of the invention having wire wound on its surface in the form of a coil.


The drawing represents a specific coil form of the invention for purposes of illustration and includes an elongated thin-walled structure 10 defining a passage 10a therethrough. The wall thickness is designated 10b. The outer peripheral surface 10c of the structure 10 forms a support for a wire coil 12 wound thereon.


In a compression molding process, a polyimide release film is placed in the steel mold having a 20 mil shim plate. On this release film 4.5 gm of pre-dried flame retarded polyethylene terephthatlente polymer pellets are spread which are then alternately covered by 6-plies of style S-120 fiber glass woven fabric weighing 39 gm. The pellet-fabric sandwich structure is built successively until all of the 27 gm of the polymer has been used. A polyimide release film is placed on the top layer and mold is closed. Next, the molding is carried out according to the following time-temperature cycle:

The mold was inserted into the press at 310° C. and held for 25 min (platens touching) under contact pressure, then held at 15 min at 50 psi, then held at 15 min at 250 psi and cooled under pressure.

A molded panel, made according to the procedure outlined above, had a void-content, as measured by C-Scan, of less than 5% indicating good consolidation. A sample of the panel was tested in accordance with UL Standard for Safety 1446, analyzed at Underwriters Laboratories, Inc. and received a Provisional Recognition as a Class R (220° C.) motor or transformer insulation.


In Example 2 the fiber-glass fabric/thermoplastic polymer is replaced with a prepreg consisting of 4-plies of B-staged epoxy impregnated aramid paper. The paper was made from 85/15 Kevlar®/Nomex® aramid fibers (about 1/4" long) on a standard wet-lay papermaking machine. The following molding cycle is used:

2 hrs at 190° C. and 250 psi

cool to room temperature under pressure

A molded panel, made according to the procedure outlined above, had a void content, as measured by C-Scan, of less than 5%, indicating good consolidation. A sample of the panel was tested in accordance with UL Standard for Safety 1446, analyzed at Underwriters Laboratories, Inc. and received a Provisional Recognition as a Class R (220°) motor or transformer insulation.

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Referenced by
Citing PatentFiling datePublication dateApplicantTitle
US5036580 *Sep 21, 1990Aug 6, 1991E. I. Du Pont De Nemours And CompanyProcess for manufacturing a polymeric encapsulated transformer
US5112901 *May 14, 1990May 12, 1992Basf AktiengesellschaftPolyether ketone polymers, creep resistance
US5534839 *Apr 5, 1995Jul 9, 1996Cramer Coil & Transformer Co., Inc.Miniature transformer
US6279807Oct 18, 1999Aug 28, 2001Roll Systems, Inc.Method and apparatus for pinless feeding of web to a utilization device
US6445269 *Sep 4, 1996Sep 3, 2002E.I. Du Pont De Nemours And CompanyDry-type high-voltage winding
US6626343Sep 17, 2002Sep 30, 2003Roll Systems, Inc.Method and apparatus for pinless feeding of web to a utilization device
US6710476 *Jul 11, 2001Mar 23, 2004Twinbird CorporationElectromagnetic reciprocal drive mechanism
US6980076May 19, 2000Dec 27, 2005Mcgraw Edison Companyinsulation paper includes wood pulp fiber, synthetic fiber, and binder material; synthetic fiber is present in paper in an amount of 2-25 percent by weight; use in electrical devices, such as transformers
US8456266Jun 21, 2010Jun 4, 2013Engineered Products Of Virginia, LlcTransformer coil assembly
WO1991014275A1 *Feb 12, 1991Sep 15, 1991Du PontProcess for manufacturing a polymeric encapsulated transformer
WO2010151548A1 *Jun 22, 2010Dec 29, 2010Engineered Products Of Virginia, LlcTransformer coil assembly
U.S. Classification428/36.1, 428/36.2, 336/198, 428/920, 336/208, 442/117, 428/417, 428/401, 442/136, 428/474.7, 428/473.5, 428/36.3, 428/902, 428/34.5
International ClassificationH01F5/02, H01B3/30
Cooperative ClassificationY10S428/92, Y10S428/902, H01F5/02
European ClassificationH01F5/02
Legal Events
Sep 24, 2002FPExpired due to failure to pay maintenance fee
Effective date: 20020731
Jul 31, 2002LAPSLapse for failure to pay maintenance fees
Feb 20, 2002REMIMaintenance fee reminder mailed
Dec 15, 1997FPAYFee payment
Year of fee payment: 8
Dec 16, 1993FPAYFee payment
Year of fee payment: 4
Jan 10, 1989ASAssignment
Effective date: 19881007